1. Field of the Invention
The invention relates to a calibrating reflector device for an optical measuring system which has an optical fiber optics probe, with a housing that is open on one side, a reflecting device with a specific reflection behavior and a positioning device, which places the front end of the fiber optics probe in the housing of the reflecting device.
2. Description of Related Art
A calibrating reflector device of the type to which the present invention relates is known from U.S. Pat. No. 4,796,633.
Calibrating reflector devices are used to calibrate optical measuring systems which are used to measure parameters of a sample, which can be detected by spectroscopic differences in the sample. These include, for example, catheter oxygen measuring systems, which are used to measure the oxygen saturation of the blood. To measure the oxygen saturation of the blood in vivo, the fiber optics probe is placed in the circulatory system of the patient.
In such optical measuring systems, the fiber optics probe comprises, for example, a thin tube with two optical waveguides whose measuring-side end is cut off vertically and polished. The light coming from a light source, going through an optical waveguide when measuring, strikes the sample to be measured and is scattered by the sample. The scattered light is recollected and evaluated for measurement. The measurements take place with light of various wavelengths, and, for example, the oxygen saturation of the blood is determined by a ratio formation of the measured values of various wavelengths.
Optical measuring systems of this type have to be calibrated, since the systems age over time and a measured value drift occurs. The reason for this lies, e.g., in such facts as that water accumulates in the plastic of the optical waveguide, that transmission changes occur, that the reproducibility of the connector between the fiber optics probe and the light source to be connected or the evaluating device to be connected changes, or that the light-sending diodes of the light source age.
Without a calibration of the optical measuring system, no absolute measurements can be obtained, only relative values can be measured. A measurement of absolute values requires a readjustment of the optical measuring system to a standard. For this purpose, calibrating reflector devices are used, which represent such a standard and are used before the actual measurement to calibrate the optical measuring system. To calibrate the optical system, the optical fiber optics probe is placed in the calibrating reflector device, which has a specific fixed reflection behavior. Then, the measuring system is calibrated from the results of measurements obtained, for example, by adjusting the obtained measuring signal amplitude on the measuring device. After calibration with a calibrating reflector device, which is used only once, the actual measurements can take place.
Calibrating reflector devices of the usual type of construction can be subdivided into so-called solid-state reflectors and so-called cavity reflectors.
An example of a known cavity reflector is the initially mentioned calibrating reflector device, which is known from U.S. Pat. No. 4,796,633. In this known device, reflecting particles are embedded in the front surface of an inside wall of the housing, and the front end of the fiber optics probe is placed at a predetermined distance opposite this front surface. For calibration, light of specific wavelengths is thrown by the fiber optics probe onto the front surface having the reflecting particles and the light reflected from the surface is absorbed and used for calibration.
However, in such a cavity reflector, the optical properties are influenced by the surface condition of the cavity walls, i.e., the inner walls of the housing, the surface condition of the light exit surface on the front end of the fiber optics probe and the exact positioning of the fiber optics probe in the housing. Since such reflectors are produced as a unit in an injection molding process, the optical properties are, moreover, influenced by the distribution, shape and size of the reflecting particles, which are embedded in the housing wall.
An example of a known solid-state reflector can be found in U.S. Pat. No. 4,322,164. In this solid-state reflector, a solid element is provided inside a housing, in which light-dispersing particles are embedded and cause a clouding, so that this solid-state element has a specific known reflection behavior and forms the measuring standard. To assure a reliable contact between the surface of this solid-state element and the front end of the fiber optics probe, the solid-state element is mounted in the housing to be axially resiliently displaceable, i.e., in the axial direction of the fiber optics probe. Additionally, the opposite side of the solid-state element, relative to the fiber optics probe, is provided with a pretensioning device, for example, in the form of a spring, by which the solid-state element is pressed firmly against the front end of the fiber optics probe during calibration.
However, in such solid-state reflectors, the optical properties of the reflector are influenced by the distribution, shape and size of the reflecting particles in the solid-state element. These parameters can be poorly controlled in the production of the reflector. The reflecting particles on the surface of the solid-state element, because of the spring pretensioning, are in a constant firm contact with the front end of the fiber optics probe, so that the danger exists that the particles detach themselves from the solid-state element. Moreover, it is not possible to easily change the type of particle since, as a rule, a change of the production process is necessary to do so. As a result, the variability of the reflection behavior in such reflectors is small.
In the solid-state reflector described in U.S. Pat. No. 4,322,164, stray reflections, especially on the coupling surface of the front end of the fiber optics probe, which occur, for example, in cavity reflectors can be avoided by the resilient pressing of the solid-state element against the front end of the fiber optics probe; however, the firm contact produced between the probe and solid-state element causes the particle distribution to be at least partially disturbed.